The present invention relates to composite materials, and more particularly to processes for fabricating composite materials that comprise a reinforcement fabric infiltrated with a powder-containing resin.
FIG. 1 schematically represents a high-bypass turbofan engine 10 of a type known in the art. The engine 10 is schematically represented as including a fan assembly 12 and a core engine 14. The fan assembly 12 is shown as including a composite fan casing 16 and a spinner nose 20 projecting forward from an array of fan blades 18. Both the spinner nose 20 and fan blades 18 are supported by a fan disc (not shown). The core engine 14 is represented as including a high-pressure compressor 22, a combustor 24, a high-pressure turbine 26 and a low-pressure turbine 28. A large portion of the air that enters the fan assembly 12 is bypassed to the rear of the engine 10 to generate additional engine thrust. The bypassed air passes through an annular-shaped bypass duct 30 and exits the duct 30 through a fan nozzle 32. The fan blades 18 are surrounded by a fan nacelle 34 that defines a radially outward boundary of the bypass duct 30, as well as an inlet duct 36 to the engine 10 and the fan nozzle 32. The core engine 14 is surrounded by a core cowl 38 that defines the radially inward boundary of the bypass duct 30, as well as an exhaust nozzle 40 that extends aftward from the core engine 14.
The fan nacelle 34 is an important structural component whose design considerations include aerodynamic criteria as well as the ability to withstand foreign object damage (FOD). For these reasons, it is important to select appropriate constructions, materials and assembly methods when manufacturing the nacelle 34. Various materials and configurations have been considered, with metallic materials and particularly aluminum alloys being widely used. Composite materials have also been considered, such as epoxy laminates reinforced with carbon (graphite) fibers or fabrics, as they offer advantages including the ability to be fabricated as single-piece parts of sufficient size to meet aerodynamic criteria, contour control, and reduced weight, which promote engine efficiency and improve specific fuel consumption (SFC).
Aircraft engine nacelles are subject to icing conditions, particularly the nacelle leading edge at the inlet lip (42 of FIG. 1) while the engine is on the ground and especially under flight conditions. One well known approach to removing ice buildup (de-icing) and preventing ice buildup (anti-icing) on the nacelle inlet lip 42 has been through the use of hot air bleed systems. As an example, engine-supplied bleed air can be drawn from the combustion chamber 24 through piping (not shown) to the inlet lip 42, where the hot bleed air contacts the internal surface of the inlet lip 42 to heat the lip 42 and remove/prevent ice formation. As an alternative, some smaller turbofans and turboprop aircraft engines have utilized electrical anti-icing systems that convert electrical energy into heat via Joule heating. Resistance-type heater wires can be used as the heating element, though a more recent example uses a flexible graphite material commercially available under the name GRAFOIL® from GrafTech International Holdings Inc. The heating element is embedded in a boot, such as a silicon rubber, which in turn is attached to the inside leading edge of the nacelle inlet lip 42. In either case, uniform and efficient heating of the inlet lip 42 can be promoted if the lip 42 is constructed of a metallic material, such as an aluminum alloy, in comparison to a composite material. To promote uniform heating of an inlet lip 42 fabricated from a composite material, such as a carbon-reinforced (fiber and/or fabric) epoxy laminate, the composite material can be produced to contain conductive fillers capable of promoting its thermal conductivity. Such fillers have included boron nitride (BN), alumina (Al2O3), and aluminum nitride (AlN) powders and carbon (graphite) nanotubes.
Traditional approaches for incorporating fillers involve admixing the filler particles into the resin system, and then infusing a carbon fabric with the particle-laden resin system. Though effective, the resulting resin system tends to have a relatively high viscosity, which limits the concentration of filler that can be incorporated into the composite. This limitation is due in part to a filtering effect, in which the reinforcement fabric filters the filler particles out of the resin system during the infusion process. To reduce this filtering effect, nano-sized filler particles may be used, though filler loadings of less than 20 volume percent are still typical due to a sharp increase in the viscosity of the resin system. As a result, through-thickness thermal conductivities of inlet lips fabricated from composite materials, such as a carbon fabric-reinforced epoxy laminates, have been limited, typically to thermal conductivity values of about 0.6 W/mK or less.
Mechanical properties, including interlaminar toughness and compression modulus, can also be limited as a result of high-viscosity resin systems tending to promote the occurrence of dry spot defects in composites due to nonuniform infusion of the reinforcement fabric. High-viscosity resin systems also limit the processes by which the resin system can be infused into a reinforcement fabric. For example, it is very difficult to uniformly infuse resin systems containing about 15 volume percent of nano-sized filler particles using such relatively low-cost processes as vacuum-assisted resin transfer molding (VaRTM).
In view of the above, it would be desirable if a method existed by which greater amounts of filler particles could be incorporated into a resin-infused reinforcement fabric. In particular, such a capability would be beneficial for incorporating greater quantities of conductive fillers in fabric-reinforced polymer composites used in the fabrication of fan nacelles. Such a capability would also be beneficial for a variety of other applications, for example, electrical enclosures and aircraft wing/tail regions fabricated from fabric-reinforced polymer composites, in which case the fillers may serve to improve thermal conductivity, electrical conductivity (such as graphite-type fillers), and/or improve interlaminar toughness (such as thermoplastic tougheners in the powder form).